(1) Field of the Invention
The present invention relates to a printing control system in a laser printer for detecting a start pulse, and printing by actuating a laser beam source with a video signal synchronized with the start pulse, for controlling the laser printer when the start pulse is not detected.
(2) Description of the Related Art
Laser printers are very popular as electrophotographic printers. As is well known, a laser printer typically carries out the processes of: producing a uniform distribution of electrical charges on the surface of a rotary photosensitive drum; forming an electrostatic latent image on the electrically charged surface of the photosensitive drum by optically writing an image thereon with a laser beam scanner comprised of a rotating polygon mirror and a laser beam source; visually developing the electrostatic latent image with a developer, i.e. toner, which is electrically charged to be electrostatically adhered to the electrostatic latent image zone; electrostatically transferring the developed visible image or toner image from the drum to a sheet or pater; and fixing the transferred image on the sheet or paper.
In these laser printers, a gas-laser beam tube having a large output power is used as a laser beam source for a high speed laser printer. However, the laser beam from the gas laser beam tube may be instantaneously cutoff for example, for as much as 10 .mu.s, due to the exisstance of a small obstacle of approximately 10 .mu.m diameter in the gas laser beam tube.
If an instantaneous cutoff occurs, it will be impossible to detect the start pulse which starts reading out of the video memory, and it is impossible to carry out printing, so that a countermeasure for an instantaneous cutoff of the laser beam is considered necessary.